1. Field of the Invention
This invention relates to safe operation of wet oxidation systems using pure oxygen or oxygen enriched gases.
2. Description of the Prior Art
Wet oxidation is a well-established process for treating aqueous wastewaters, sludges and slurries which contain oxidizable substances; more than one hundred wet oxidation units are in commercial operation. Many patents and other publications disclose wet oxidation processes using air as the source of oxygen for accomplishing the oxidation. Schoeffel U.S. Pat. Nos. 3,042,489 and 3,097,988, and Pradt et al. U.S. Pat. No. 3,654,070 disclose the application of pure oxygen or an oxygen enriched gas to wet oxidation processes. For this discussion, the term "oxygen", when used without a modifying adjective will refer to any gas containing greater than 21 mole percent oxygen, to distinguish it from air.
Increased reaction rates and the opportunity to operate at lower pressures and temperatures make the use of oxygen very attractive from a theoretical standpoint. In addition, many potential users of the wet oxidation process, such as sewage treatment plants, steel mills, etc. have existing oxygen generation/storage facilities, making the gas available at low cost.
To date however, no wet oxidation processes have been operated commercially using oxygen. One important reason is that no one has yet shown how oxygen can be safely used in wet oxidation processes under steady state and transient conditions common to such processes.
In wet oxidation systems, aqueous and gaseous phases coexist at elevated pressures and temperatures. System pressures are chosen so that there will always be an aqueous phase. Oxidation reactions consume oxygen and generate carbon dioxide. When the aqueous phase has a neutral or low pH, a major portion of the carbon dioxide formed by wet oxidation will remain in the gaseous phase, diluting the oxygen. When the aqueous phase is caustic, however, much of the carbon dioxide will be absorbed in the aqueous phase.
The quantity of water vapor which is present in the gas phase is a function of temperature, pressure, and quantity of non-condensible gases (NCG), and can be determined by known thermodynamic relationships. For a given system operating at a nearly uniform pressure, the degree of gas dilution by water vapor is much greater at the higher temperatures.
In prior art processes using air as the source of oxygen, the percentage of oxygen in the gas phase at elevated temperatures and pressures is considerably less than 21 percent, even without any oxygen consumption. For example, at 550.degree. F. and 1000 psi pressure, water vapor dilutes the oxygen from its original 21 percent to a concentration of about 5 percent. As oxygen is consumed its concentration at reactor conditions drops to very low values. Therefore, pure oxygen or oxygen enriched gas can be used advantageously in enhancing the rate and completeness of oxidation, so long as the safety of the process can be ensured.
Gaseous oxygen, when diluted to a concentration of 21 mole percent as in the form of air, is safe to handle, even when compressed to quite high pressures.
However, oxygen at higher concentrations, especially high purity oxygen, is likely to undergo rapid, spontaneous combustion when placed in contact with organic or other oxidizable substances at pressures above atmospheric, even at room temperature. In the wet oxidation process, high concentrations of oxidizable materials are deliberately oxidized. It is vital to control the process so that transient excursions of temperature, pressure, and thermal efficiency are minimal and hazardous operating conditions do not occur.
Moreover, many metals such as steel, aluminum and titanium, for example, will burn vigorously in the presence of oxygen once an ignition has occured. Titanium itself has been shown to be capable of undergoing spontaneous combustion under certain conditions in the presence of oxygen and water at elevated pressures, as reported by F. E. Littman and F. M. Church in Final Report: Reactions of Titanium with Water and Aqueous Solutions, Stanford Research Institute Project No. SD-2116, June 15, 1958.
In the handling of oxygen, traditional safety practice has emphasized selection of materials of construction which will not themselves undergo spontaneous combustion at design operating conditions, and strict cleanliness standards to ensure that no contaminants capable of spontaneous combustion are present in the system. In wet oxidations, however, the choice of materials of construction is nearly always constrained by the corrosive properties of the wastewater, sludge, or slurry being oxidized. Thus titanium or titanium alloys may be dictated as the material of construction when severe corrosion of iron- or nickel-based alloys is indicated. Moreover, the wet oxidation system treats wastewaters, sludges, or slurries which may contain up to ten percent or even higher concentrations of organic substances, and its interior surfaces may always be contaminated with substances capable of spontaneous combustion upon contact with oxygen at high pressures.
Therefore, the use of wet oxidation employing pure oxygen or an oxygen enriched gas in a system fabricated of titatium, where the interior surfaces may always be contaminated with organic matter appears to be definitely precluded.
On the other hand, the use of oxygen in a wet oxidation installation may be very attractive. If oxygen is already available onsite, the capital and operating costs for a large air compressor are eliminated. Favorable oxidation kinetics will result in a smaller reactor and/or lower operating temperature and pressure. Other potential advantages of using oxygen may be evident to those familiar with wet oxidation.
The object of this invention, therefore is to make possible the use of pure oxygen or oxygen enriched gas in a wet oxidation process under conditions of safety comparable to traditional wet oxidation processes using air.